A conventional fan scavenged two-stroke internal combustion engine comprises piston adapted for reciprocatable vertical movement in a cylinder. The piston is attached by a connecting rod to a crankshaft which rotates in a housing or crankcase located below the cylinder and which is in open communication therewith. At the start of the first stroke the piston is in its highest position and when a mixture of compressed petrol (gasoline) and air above the piston is ignited the piston is thrust downwardly and in so doing releases an exhaust port in the cylinder. The burned gases in the cylinder which are still under high pressure can thus escape through this port. When the piston descends further its upper surface may release an inlet port in the cylinder which admits fresh petrol and air moisture into the cylinder so that the remaining burned gases are flushed out. When the piston rises again in its second stroke both exhaust port and inlet ports are closed for a time and during this period the petrol and air mixture is compressed so that a fresh cycle can commence. A fan is located in or adjacent to the inlet port to force fresh petrol and air mixture into the cylinder.
A conventional crankcase scavenged two stroke internal combustion engine has no scavenging fan. Instead the crankcase is hermetically sealed so that it can function as a pump in conjunction with the piston. When the piston ascends, a partial vacuum is produced in the crankcase until a lower surface of the piston releases an inlet port and this opens the way for fresh petrol and air mixture into the crankcase. When the piston descends, the mixture in the crankcase is compressed slightly so that as soon as a top surface of the piston releases a transfer port or overflow duct in the cylinder which connects the crankcase to the cylinder the compressed mixture may enter the cylinder through the transfer port above the piston.
In the fan scavenged two stroke engine the fan adds to the cost. However as the transfer port between the cylinder and crankcase is eliminated, the crankshaft can be provided with forced-oil lubrication without involving a risk that oil in the crankcase can find its way in the cylinder In the cheaper crankcase scavenged engine the lubricating oil is mixed with petrol or alternatively is supplied to the points of lubrication dropwise by small lubricating oil pumps The oil which enters the crankcase is liable to be carried through the transfer port into the cylinder whence it passes through the exhaust port.
Disadvantages of conventional two stroke engines of the type referred to above include
(1) because the crankcase is in open communication with the cylinder the burnt gases arising out of combustion can be corrosive to the cylinder and crankcase. Also impurities can gain access to the cylinder or crankcase causing wear and tear on bearing surfaces causing frequent maintenance checks, PA1 (2) the connecting rod is pivotally connected to both crankshaft and piston thus causing friction between piston and cylinder and undue wear and tear on pivotal interconnections between crankshaft and connecting rod and piston and connecting rod, PA1 (3) excessive number of moving parts; and PA1 (4) the thrust transmitted to the crankpin which pivotally interconnects the connecting rod and crankshaft is not in a straight line and thus power losses may occur. PA1 (a) a single piston housing; PA1 (b) a transfer port; PA1 (c) an inlet port in the housing; PA1 (d) an exhaust port in the housing; PA1 (e) a piston located in the housing; PA1 (f) a connecting rod integral with or otherwise rigidly secured to the piston; PA1 (g) a crankcase located adjacent to the piston housing but sealed therefrom; PA1 (h) a crankshaft in the crank case; and PA1 (i) pivotal connection means between the crankshaft and connection rod to facilitate rotation of the crankshaft in the crankcase; and PA1 (j) electrical ignition means located within the crankcase.
Reference may be made to two-stroke cycle spark ignition engines employing a scotch-yoke crankshaft mechanism which are described generally in a paper by David Kirk in SAE Technical Paper Series 851518 based on the 1985 International Off-Highway and Powerplant Conference and Exposition held at Mecca, Milwaukee, Wis. USA on Sept. 9-12, 1985.
In this paper reference is made to a two stroke cycle spark ignition engine intended for light aircraft propulsion. A modified scotch yoke crank mechanism is employed and by two such reciprocating assemblies set at right angles to each other a compact 4-cylinder radial engine configuration results. The conventional scotch yoke mechanism as described in the above reference therefore included two identical tubular piston rods having integral heels attached by bolts to a yoke member which may therefore reciprocate back and forth. A crankshaft was rotatably attached to a slipper such as a roller block slidably mounted in the yoke. Upon reciprocatable movement of the piston rods rotation of the crankshaft in the crankcase was therefore effected. The Kirk engine is provided with a single magneto located on the exterior of the engine and driven directly from the, accessory end of the crankshaft.
Reference is made to Albertson U.S. Pat. No. 3,617,652 which also described various forms of scotch yoke mechanisms which however are all based on the general principle as described above. U.S. Pat. Nos. 981,995; 1,181, 892; 2,122,676; 2,122,677; 2,172,670; 2,147,666; 4,013,048; 4,331,108 and French Pat. No. 595,04 are also mentioned in the aforementioned Kirk reference but are not believed relevant to the present invention. Of the aforementioned references Reitz U.S. Pat. No. 4,013,048 is the most interesting reference as it also described a modified Scotch crank mechanism.
However, there has long been a requirement for a two stroke engine based on the Scotch crank principle which could be applied to a single piston and cylinder arrangement which is most efficacious in regard to lawn mower engines which usually require a manual pull start. However as set out above, most lawn mower engines have been manufactured on the conventional fan scavenged or crankcase scavenged arrangement as described above with the consequential disadvantages.